The detection and interdiction of illicitly trafficked Special Nuclear Material (SNM) is very important in the ongoing anti-terrorist activities undertaken by homeland security agencies. U.S. Patent Application No. 2005/0105665 by Lee Grodzins and Peter Rothschild for a system of detection of neutrons and sources of radioactive material, published May 19, 2005, provides the following state of technology information: “There is a need to find sources of radiation and other nuclear material that are clandestinely transported across national boundaries. The sources of clandestine nuclear material may be in the form of “dirty bombs” (e.g., a conventional explosive combined with radioactive nuclides designed to spread radioactive contamination upon detonation), fissile material, and other neutron and radiation emitting sources that may present a hazard to the public. During recent years, the United States government has placed mobile vehicles at strategic areas with gamma ray detectors dedicated to the task of finding fissile material. Atomic explosives may be made from 235U, a rare, naturally occurring, isotope of uranium that lives almost 109 years, or 239PU, a reactor-made isotope that lives more than 104 years. 235U decays with the emission of gamma ray photons (also referred to as ‘gammas’), principally at 185.6 keV and 205.3 keV. 239Pu emits a number of gamma rays when it decays, the principal ones being at 375 keV and 413.7 keV. These gamma rays are unique signatures for the respective isotopes. But fissile material invariably contains other radioactive isotopes besides those essential for nuclear explosives. For example, weapons grade uranium may contain as little as 20% 235U; the rest of the uranium consists of other isotopes. The other uranium and plutonium isotopes reveal their presence by gamma rays emitted by their daughters. For example, a daughter of 238U emits a high energy gamma ray at 1,001 keV; a daughter of 232U, an isotope present in fissile material made in the former USSR, emits a very penetrating gamma ray at 2,614 keV; and a daughter of 241Pu emits gamma rays of 662.4 keV and 722.5 keV.”
U.S. Pat. No. 4,201,912 issued May 6, 1980 to Michael L. Evans et al and assigned to The United States of America as represented by the U.S. Department of Energy, provides the following state of technology information: “A device for detecting fissionable material such as uranium in low concentrations by interrogating with photoneutrons at energy levels below 500 keV, and typically about 26 keV. Induced fast neutrons having energies above 500 keV by the interrogated fissionable material are detected by a liquid scintillator or recoil proportional counter, which is sensitive to the induced fast neutrons. Since the induced fast neutrons are proportional to the concentration of fissionable material, detection of induced fast neutrons indicates concentration of the fissionable material.”
U.S. Pat. No. 3,456,113 issued Jul. 15, 1969 to G. Robert Keepin and assigned to the United States of America as represented by the U.S. Atomic Energy Commission, provides the following state of technology information: “An apparatus and method of detecting, identifying and quantitatively analyzing the individual isotopes in unknown mixtures of fissionable materials. A neutron source irradiates the unknown mixture and the kinetic behavior of the delayed neutron activity from the system is analyzed with a neutron detector and time analyzer. From the known delayed neutron response of the individual fission species it is possible to determine the composition of the unknown mixture. Analysis of the kinetic response may be accomplished by a simple on-line computer enabling direct readout of isotopic assay.”
A neutron is created by a physical process, either fission or an inducing nuclear reaction. The created neutron or neutrons then interact with the environment. If the environment contains more nuclear material (i.e., uranium), the first neutrons may create more neutrons by causing more fission or other nuclear reactions. The first and second and subsequent neutrons are a chain. A chain may start with an alpha particle creating a single neutron that subsequently creates hundreds of fissions. Another chain may start with a spontaneous fission creating three neutrons that go on to create hundreds of fissions. These chains evolve over time and some of the neutrons are absorbed or lost. Some members of the chain may be finally captured in a neutron detector device. The final captured neutrons may be counted as a simple sum or observed as a time dependent rate. What may start out as a chain of 1000 neutrons may result in a count of two neutrons during some period of time, in a detector.
Fission is generally defined as the emission of multiple neutrons after an unstable nucleus disintegrates. For example, Pu240 decays at a rate of about 400 fissions per second per gram of Pu240 atoms. When the fission occurs, multiple neutrons are emitted simultaneously, with the number ranging from zero to eight neutrons. This simultaneous neutron emission characteristic is unique to fission. A standard approach to locating neutron sources is to use a neutron detector to look for count rate increases above background patterns. Given the number of legitimate neutron sources used in industry, deploying standard neutron detectors will result in a large number of alarms that will need to be resolved by more intrusive inspection techniques.
Systems have been developed to identify when fission is occurring in an unknown or suspected dangerous source by providing an analysis of the range of simultaneous neutrons relative to standard (e.g., Poisson) distribution curves. Such systems employ methods of counting neutrons from the unknown source and detecting excess grouped neutrons to identify fission in the unknown source. Such systems may also include methods of plotting a Poisson count distribution on top of a measured count distribution, such that the mean count of the data is the same as that of the Poisson curve, and discerning differences attributed to fission in the unknown source. The objective of such methods is to assess a measurement to determine if there is correlation of emitted neutrons in excess of a background (or naturally occurring) level of neutrons.
In general, there is an obvious case where a neutron detector has a high count rate relative to the typical low background count rate because of the fact that there must be some kind of neutron source present otherwise the detected count rate would be near or at the expected count rate for the background count rate. Background count rates can vary, however, due to variations in time and environmental conditions. In this case, a simple comparison of detected versus background rates may result in erroneous data.
What is needed, therefore, is a method of conditionally assessing the excess in correlation of an unknown measurement compared to the correlation present in a data defined as background. What is further needed is a technical definition of excess correlation intended to properly handle the measured excess correlation, unlike the error of simply subtracting an erroneously defined background rate, or subtracting the second moments derived from two measurements, or subtracting mass equivalents derived by some theory about how correlation relates to effective mass of decaying nuclear material.